But as we have said, everything which moves is moved, being movable in capacity, and of which the immobility is rest: since the immobility of that to which motion is present is rest. For to energize with respect to that which is movable, so far as it is movable, is to move. But it effects this by contact: so that at the same time also it suffers. Hence motion is the energy of that which is movable, so far as it is movable; and this happens from the contact of that which is motive; so that at the same time, it likewise suffers. But that which moves always introduces a certain form, either this particular thing, or of such a quality, or of a definite quantity, which will be the principle and cause of the motion when it moves. Thus, man in energy makes man from man, which is incapacity. That also which is the subject of doubt is evident, namely, that motion is in the movable thing: for it is the energy of this, and is derived from that which is motive. The energy likewise of that which is motive is not different. For it is necessary that both should be energy; since it is motive because it has the capacity of being so; and moving, because it energizes. But it is energetic of the moveable; So that in a similar manner there is one energy of both. Just as there is the same interval of one with respect to two, and of two with respect to one; and also between acclivity and declivity: for these are one but the definition is not one the like also takes place in the mover, and that which is moved. There is here, however, a logical doubt: for, perhaps, it is necessary that there should be a certain energy different from that which is effective, and that which is passive; since the one is a producing or making, but the other is passion. But with respect to work and end, of this indeed it is effect, but of that passion. Since, therefore, both are motions, if they are different, in what will they be inherent? For either both are in that which suffers and is moved, or the making is in the maker, but the passion in the patient. But if it be necessary to call this also making, it will be homonymous. If, however, this be the case, motion will be in that which moves: for there is the same reason of that which moves, and of that which is moved. Hence either everything which moves will be moved, or that which possesses motion will not be moved. But if both action and passion are in that which is moved and suffers, and also teaching and discipline, which are two, are in the learner, then in the first place, the energy of each thing will not be in each; and in the next place, it is absurd that two motions should be at once moved. For there will be certain mutations of quality, two of one subject, and to one form, which is impossible. Will there be one energy? But it is absurd that there should be one and the same energy of two things different in species. There will be, however, if discipline and teaching, action, and passion are the same. To teach also will be the same as to learn, and to act as to suffer; so that it will be necessary that every teacher should learn, and every agent suffer. Or shall we say that neither is it absurd that the energy of one thing should be in another? For teaching is the energy of the teacher; yet it is in something, and is not cut off, but is of this in this. Nor is it absurd that there should be one and the same energy of two things, not as if the essence were the same, as in an upper and an under garment; but as that which is incapacity to that which energizes. Nor is it necessary that the teacher should learn, not even if to suffer and to do were the same: since they would not be the same, as if there were one definition declaring the essence of the thing, as in an upper end an under garment; but they would be the same, just as the way from Thebes to Athens is the same as the way from Athens to Thebes, as was before observed. For not all the same things are inherent in things which are in any way whatever the same, but only in those of which the essence is the same. It does not follow, however, that if teaching is the same as discipline, to learn is the same as to teach; as neither if there is one distance of things which are distant, is it one and the same thing for this to be distant from that, and that from this. In short, neither is teaching properly the same as learning, nor acting as being passive; but the motion in which these are inherent is the same: for to be the energy of this in this, and of this from this, differ in definition.
